In 3GPP (3rd Generation Partnership Project), the cellular packet-switched communication systems HSPA (High Speed Packet Access) and LTE (Long Term Evolution) have been specified for radio transmission of data packets between user terminals and base stations in a cellular/mobile network. Transmissions from the base station to the user terminal is referred to as “downlink” DL and transmissions in the opposite direction is referred to as “uplink” UL. In the following description, “terminal” is used to generally represent any user equipment (commonly referred to as UE in the above systems) capable of wireless communication, e.g. with base stations in a cellular/mobile network.
There are two basic modes of operation available for wireless transmissions: FDD (Frequency Division Duplex) and TDD (Time Division Duplex). In FDD, downlink and uplink transmissions are made at separate frequency bands, such that data can be transmitted in the downlink and uplink at the same time without mutual interference. In TDD, on the other hand, downlink and uplink transmissions are made on the same frequency band and must therefore be separated in time to avoid interference.
The TDD operation mode is flexible in that the duration of downlink and uplink transmissions can be configured depending on the traffic intensity in the respective downlink and uplink directions, thus allowing for connections with asymmetric transmission schemes. In a cellular system with multiple cells, each cell being served by a base station, interference between uplink and downlink transmissions should be avoided. Therefore, the base stations are typically coordinated for synchronized operation where the uplink and downlink periods of the cells in the same area occur simultaneously. For asymmetric connections with downlink intensive traffic, the downlink time period may be configured greater than the uplink time period, and vice versa for connections with uplink intensive traffic.
For LTE, a new physical layer is currently being standardized in 3GPP that is based on OFDM (Orthogonal Frequency Division Multiplexing) in the downlink and SC-FDMA (Single Carrier Frequency Division Multiple Access) in the uplink. The new physical layer shall support both FDD and TDD operation, and there should be a high degree of commonality between these two modes of operation. The SC-FDMA properties in the uplink require that any data transmitted from each terminal basically maintains single carrier properties.
The transmissions in both FDD and TDD operation are generally scheduled in radio frames, and each radio frame is typically divided into multiple sub-frames. In the following description, the term “sub-frame” is used to generally represent a predefined transmission time interval (sometimes referred to as “TTI”) or time slot, in which information can be transmitted in the form of “data blocks”, although not limited to any particular standard or duration. Blocks of data can thus be transmitted in each sub-frame. For example, a base station may transmit data blocks to one or more terminals in each sub-frame, and a terminal can be assigned resources for a data block in each downlink sub-frame. Further, one or more terminals can transmit data blocks in assigned resources in uplink sub-frames to the base station.
In LTE, the predefined radio frame is 10 ms (milliseconds), which is divided into ten predefined sub-frames of 1 ms duration each. In the FDD mode, where data can be transmitted in the downlink and uplink simultaneously, there are 10 downlink sub-frames “DL” and 10 uplink sub-frames “UL” available during one radio frame on separate frequency bands F1 and F2, respectively, as illustrated schematically in FIG. 1a. In the TDD mode, there are in total ten downlink and uplink sub-frames available for data transmission during one radio frame, which can thus be transmitted only one at a time on a common frequency band F. In general, guard periods are needed to separate uplink sub-frames from downlink sub-frames, and one or two downlink sub-frames may therefore be somewhat shorter which could be considered as downlink parts of time slots or sub-frames, and there may also be certain uplink time slots not used for data, which is however not necessary to describe in more detail to understand the present invention.
As mentioned above, downlink and uplink transmissions can be configured in TDD on a cell basis depending on the traffic demands in either direction. For example, the downlink/uplink allocation can be configured to eight downlink sub-frames and two uplink sub-frames during one radio frame on the same frequency band F, as illustrated schematically in FIG. 1b. Another possible configuration could be 5 DL:5 UL sub-frames, and yet another configuration could be 7 DL:3 UL sub-frames. The alternation pattern of downlink/uplink sub-frames can also be configured optionally. For example, the downlink/uplink sub-frame pattern in FIG. 1b could be modified into eight successive downlink sub-frames followed by two uplink sub-frames.
A base station may transmit data blocks in downlink sub-frames to one or more terminals, and the terminals transmit data blocks in uplink sub-frames to the base station. More specifically, the base station may transmit commands in each downlink sub-frame to the terminals that data blocks are allocated for them in the current downlink sub-frame. The base station could also transmit a more persistent allocation with a pattern of downlink allocations to a terminal, so that it may, e.g., receive a data block every 20 ms.
The transmission in either direction is typically subjected to various disturbances, including propagation fading and interference from reflections and other transmissions, such that errors may have been introduced in the data blocks when received. Thus, the channel between a base station and a terminal is often referred to as a “lossy” channel. Errors may also arise due to a poor receiver and/or antenna.
When receiving a data block in a sub-frame, the receiver in the terminal (or in the base station) is configured to check as to whether any errors are present in the received data block. A common method of detecting errors involves calculation of a check-sum or the like, which is well-known in the art. To enable correction of such errors, the data sending party must retransmit any erroneously received data block, unless some error correction mechanism can be applied successfully at the data receiving party. Therefore, the data receiving party is typically obliged to send a feedback report to the data sending party for each received data block or sub-frame, indicating if the data block was basically received correctly (i.e. without errors) or not. In LTE, for example, when certain forms of multiple antenna transmission are used, a single terminal can also receive two data blocks in the same sub-frame, each data block requiring a feedback report. In that case, the terminal is thus obliged to transmit feedback reports for both data blocks.
If the data block was received correctly, the data receiving party sends an acknowledgement “ACK”, and if the data block contained errors, it sends a negative acknowledgement “NACK”. Although the terms ACK and NACK are frequently used in this description, any equivalent or similar messages may be used for feedback reports and the present invention is not limited in this respect. “Feedback report” is used in the following as a generic term for such ACK/NACK messages and their equivalents, and one feedback report is basically needed for each received data block.
Both HSPA and LTE employ a HARQ (Hybrid Automatic Repeat ReQuest) protocol in their respective MAC (Medium Access Control) layers. The basic functionality of the processes defined in the HARQ protocol is to correct any erroneously received data blocks by means of retransmission based on the above-described feedback reporting mechanism. In this context, a feedback report is sometimes called “HARQ status report”.
For example, the data receiving party can simply discard an erroneously received packet. In more advanced solutions, the receiving party stores the signal representing the erroneously received packet in a buffer and combines this stored information with the retransmission. This is often referred to as “HARQ with soft combining” which can be used to increase the probability of correctly decoding the transmitted packet. In HARQ with soft combining, the pattern of coded bits in a particular packet may differ between transmission and retransmission, although they must obviously represent the same information.
The HARQ process is used to associate a potential retransmission to its original transmission in order to enable the soft combining at the data receiving party. When the receiving party has reported correct reception of data sent on a HARQ process, that Process can be used to transmit new data. Consequently, before the reception of a HARQ status report from the receiving party, the data sending party does not know whether it should transmit new data or retransmit the “old data”. In the meantime, the sending party therefore “stops and waits” until the result of the transmission is reported. In order to still be able to utilize the link during these waiting periods, multiple parallel HARQ processes can be applied which allows for continuous transmission.
For example, when a data block is transmitted on the downlink, the receiving terminal checks for errors in the data block and sends a feedback report to the base station. If the base station then detects a NACK, it can retransmit the information in the data block. This mechanism can also be used for data blocks sent on the uplink. In LTE, the feedback required for HARQ with soft combining is conveyed by a single bit indicating either ACK or NACK. The timing relation between the data block transmission from the sending party and the feedback report transmission from the receiving party is typically used to indicate which data block the feedback report relates to.
In FDD, the number of available sub-frames is equal in the downlink and the uplink, as shown in FIG. 1a. Consequently, it is possible to send a feedback report for one received downlink sub-frame in a given uplink sub-frame according to a “one-to-one relation”, using a fixed time interval between reception and feedback. Thereby, the data sending party can derive which HARQ process a received feedback report refers to, based on which sub-frame the report was received in. Thus, for FDD, the feedback reports for data blocks received in a downlink sub-frame n are always transmitted in uplink sub-frame n+k, where k corresponds to the processing delay in the terminal which has been agreed as k=4 for LTE FDD. Further, if uplink resources have been allocated for a terminal in the corresponding uplink sub-frame, it may transmit the feedback report in a time-multiplexed fashion together with the transmitted data block. If the terminal has not been allocated any resources for data, it will use a certain control channel in that specific uplink sub-frame. Hence, the terminal is either explicitly or implicitly assigned a feedback resource in uplink sub-frame n+k.
In TDD, on the other hand, this fixed feedback scheme is not useful since when data is received in sub-frame n, sub-frame n+4 may not be an uplink sub-frame and hence no opportunity to send a feedback report. One example of this is when there are more than four consecutive DL sub-frames in the downlink/uplink sub-frame pattern. Another example is when the sub-frame pattern dictates that the next three sub-frames are uplink sub-frames but the fourth is a downlink sub-frame. A further example is when the next sub-frame is downlink, the following two ones are uplink and the fourth one is again a downlink sub-frame. Furthermore, the allocation of uplink and downlink sub-frames may be such that the number of downlink sub-frames is greater than the number of uplink sub-frames.
In the allocation example shown in FIG. 2, there are eight downlink sub-frames but only two uplink sub-frames available. Hence, feedback reports for the eight downlink sub-frames must be transmitted in the two uplink sub-frames. Depending on how many users that have been scheduled in the downlink sub-frames, the number of feedback reports that need to be transmitted may increase by a factor 4. Furthermore, if a single terminal has been scheduled to receive data in all available downlink sub-frames, that terminal will need to transmit feedback reports for a plurality of downlink sub-frames during a single uplink sub-frame.
In TDD, the above-described report mechanism with a fixed time interval cannot generally be used, since the feedback report for a received sub-frame cannot be transmitted a fixed time interval after receiving the sub-frame if the corresponding sub-frame is not available for transmission from the data receiving party. Consequently, the feedback report for that received sub-frame must be delayed at least to the first sub-frame available for transmission. Moreover, the data receiving party typically requires a certain delay after receiving a sub-frame, for processing the data therein and to determine if it was received correctly or not, before a feedback report can be sent for that sub-frame. For example, if the receiver of data needs a delay of at least one sub-frame for processing, a received sub-frame k cannot be reported until sub-frame k+2 or later. If the receiver needs three sub-frames for processing, as in LTE, then the feedback cannot be reported until sub-frame k+4, and so forth.
A straightforward and obvious solution for the timing or scheduling of feedback reports in TDD, is to specify a minimum delay period needed for processing, from the point data is received in a sub-frame until a feedback report shall be transmitted for the received data. The feedback report is then sent in the first available sub-frame for transmission in the reverse direction after the minimum delay period. Hence, if one or more sub-frames after the delay period are allocated for reception, the feedback report must be further delayed until the first sub-frame allowing transmission occurs.
However, as a result of scheduling feedback reports according to the timing solution above, a great number of feedback reports will typically be transmitted in the same sub-frame. This could also be the case even when the number of uplink and downlink sub-frames is the same with a certain periodicity. This is particularly a problem when it is desirable to reduce the number of such reports in a single sub-frame, and particularly the maximum number of feedback reports that a single terminal may need to send as a consequence of the downlink scheduling assignments.
In FIG. 2, this is illustrated by means of an example where an asymmetric connection is configured with eight successive downlink sub-frames (sub-frame 0-7) followed by two uplink sub-frames (sub-frame 8-9). In this example, the minimum delay period needed for processing is specified as one sub-frame. Following the obvious timing solution above, feedback reports for data received in sub-frames 0-6 will all be transmitted in sub-frame 8 and the feedback report for sub-frame 7 will be transmitted in sub-frame 9 after the necessary one sub-frame minimum delay, as illustrated by dashed arrows.
If the physical channel structure must be configured to handle a great number of feedback reports in a single sub-frame, and also if a single terminal needs to transmit feedback reports for multiple DL sub-frames, as in sub-frame 8 above, the channel structure will become more complex. Also, the more feedback reports to transmit from a terminal, the more feedback resources are needed, e.g., in terms of number of codes. Hence, more bits to send from a single terminal basically require more feedback resources. Furthermore, a relatively great transmission power would then also be required to obtain sufficiently low error probability when several feedback reports are transmitted simultaneously, which is a problem as the transmission power should generally be kept low considering power consumption and network interference problems.